At present, devices have appeared in the market capable of establishing a high-speed radio network system for homes and offices. Products based on IEEE 802.11b, IEEE 802.11a, and IEEE 802.11g standardized by radio LAN standard IEEE 802.11 in the United States are examples of such devices. These radio LAN standards are based on a prerequisite that the communications is carried out in an ideal wave environment having no interference. In other words, these radio LAN standards have been established without considering the present day situation where access points (AP) are installed in high density. On the other hand, IEEE 802.11h describes a closed loop type method of obtaining transmission power (EIRP) and reception power in a Request & Response format. However, the closed loop type method is for carrying out control in the environment of association, and is not for positively avoiding generation of interference.
Patent Documents 1 to 3 describe conventional techniques for avoiding generation of wave interference in a radio network system. The Patent Document 1 discloses a technique for determining a transmission output and a communication speed based on information and reception field strength included in a signal from a radio control device, determining viable transmission power and a communication speed based on a state of the radio communication device, and performing communications based on the instructed transmission output and the instructed communication speed and a viable transmission output and a viable communication speed. In other words, the Patent Document 1 discloses, in a radio communication system, a transmission power control method of interference controlling a radio communication apparatus by adding transmission power and interference information to a transmission frame using a radio control device within a communication house facility, and a closed loop type method of changing a communication speed to carry out communications within a transmission power range prescribed from a battery residual amount of the radio communication device itself.
The Patent Document 2 discloses an open loop method of avoiding a sudden increase of transmission power, i.e., preventing causing large interference to other user, by suppressing a sudden increase of transmission power when there is a large error in measuring reception power of a desired wave signal from the other station, or when a level of a received signal suddenly decreases. Specifically, a base station has a function of estimating the amount of interference with other user from a desired wave signal to interference power ratio (SIR) obtained from a reception wave from a mobile station, determining a maximum change amount of transmission power during a control cycle of transmission power control, and instructing a maximum change amount of transmission power to the mobile station, thereby controlling interference. As a result, when there is a large error in measuring reception power of a desired wave signal from the other station, or when a level of a received signal suddenly decreases, it is possible to avoid a sudden increase of transmission power, i.e., prevent causing large interference to other user, by suppressing a sudden increase of transmission power.
The Patent Document 3 discloses the following method. A transmission path attenuation amount is calculated based on a reception CNR (Carrier to Noise Ratio) estimated from equalization error power obtained from a signal transmitted from the other transmission and receiving apparatus, and based on transmission power information included in the transmission signal from the other transmission and receiving apparatus. A transmission level having a minimum power value satisfying a reception-required CNR at the other party based on a modulation method used in the own station is determined. The determined transmission level is inserted into a control information symbol of a radio frame of a transmission signal to generate a transmission signal. A transmission level of the generated transmission signal is adjusted to the transmission level determined in advance, and the adjusted transmission signal is transmitted.
Patent Document 1: Japanese Patent Application Laid-open No. 2000-217144
Patent Document 2: Japanese Patent Application Laid-open No. H11-145899
Patent Document 3: Japanese Patent Application Laid-open No. 2004-72666